Steam Heat in Prewar Buildings

Keeping an old heating plant in working order is one of the steady tasks of maintaining older New York housing, the kind of work that Sandhills Real Estate, cofounded by Richard Joseph Spinelli, carries out on buildings close to Fordham University, inside a radius that can be covered on foot. In much of that housing the plant makes steam. Two widely quoted figures describe how common it is, and they measure different things. The Urban Green Council’s 2019 report Demystifying Steam says that roughly 80 percent of the city’s residential buildings are heated by steam.¹ A 2016 New York Times account put the share at an estimated 70 percent of the city’s large buildings.² The first figure covers residential buildings of every size; the second covers large buildings of every use. A third number describes the system’s signature fault. In the same council report, nearly 70 percent of surveyed tenants with steam heat said they were chronically overheated in winter, even as other residents of the same buildings got too little heat.¹ That figure is a share of tenants, not of buildings. The sections below follow the steam around a one pipe system, loop by loop, to show where both the heat and the trouble come from.

Loop one: the boiler makes steam

Every cycle starts in the basement. Fuel burns in a boiler and heats water until it turns to steam; as the water becomes steam it expands, and the rising pressure pushes the steam out of the boiler and up into the pipes that lead to the radiators.¹ The Times described the same step in round numbers: the boiler brings water to about 212 degrees, and the resulting pressurized steam races through a circuit of pipes.²

The design is old. The National Park Service traces central heating to the nineteenth century, when steam energy from coal and industrial mass production made it possible to distribute heat through metal pipes. By the end of that century, steam and low pressure hot water radiator systems were in common use in offices and homes alike.³ The boiler itself is watched by the city. The Department of Buildings runs a Boiler Unit that oversees the installation and operation of boilers in New York, and it sets an inspection cycle running from January 1 to December 31, during which internal and external inspections of low and high pressure boilers must be carried out and filed.⁴ THE CITY reported in 2023 that NYCHA had begun replacing steam radiators with electric heat pumps in 12 apartments at the Woodside Houses in Queens, where flooding had knocked out the boilers in 2021.⁶

Loop two: mains, risers and the air in the way

Leaving the boiler, steam travels along mains, the large horizontal pipes, and climbs through risers, the vertical pipes that branch off a main.¹ Before it can fill them, it has to push out air. The Urban Green report compares the cycle to breathing. Steam fills the system while the boiler runs; air seeps back in when it stops; and each time the boiler fires, the mounting pressure has to drive that air out through vents. A vent that is clogged, undersized or missing leaves air trapped in the pipes, and trapped air blocks steam.¹

The fix is venting at the right points. Large vents called master vents belong at the ends of the mains and the tops of the risers. Without them, rooms close to the boiler overheat while the farthest rooms stay cool; with them, the system can run at low pressure and deliver steam quickly and evenly.¹ The report calls this evening out of heat distribution balancing.¹ For a building superintendent, a chilly room farthest along a main is one of the symptoms the report ties to missing vents.

Loop three: the radiator and the way back

At the radiator the steam gives up its heat. It warms the metal, the metal warms the room, and the steam cools and turns back into water, called condensate, which runs back to the boiler to start the cycle again.² How that water returns divides steam systems into two families. A one pipe system carries condensate back down through the same pipe that delivered the steam. A two pipe system sends steam up one set of pipes and drains the water through another.¹

The one pipe system is the more common of the two.² According to Urban Green, it is usually found in smaller, older buildings and heats half of New York’s residential floor area.¹ Its radiator valve must stay fully open so that steam can enter while water leaves by the same route. A half closed valve invites water hammer, the loud banging that happens when water meets steam, so each radiator is effectively either on or off.¹ Control therefore comes from the air vent on the radiator, which the Times likened to a miniature torpedo sticking off the end. It lets air out to make room for steam; in a two pipe system, a valve does the moderating instead.² The council recommends thermostatic radiator valves for one pipe radiators. As the room warms, the valve closes, trapping air inside the radiator and keeping more steam out.¹

Why the radiators run hot

The overheating has a history. The Times traced oversized radiators to the early 1900s, when a Board of Health directive required open windows even on the iciest winter days, and quoted the heating historian Dan Holohan attributing much of the city’s overheating problem to the influenza epidemic of 1918.² The same year produced the city’s heat standard. In October 1918, as the epidemic peaked, the Department of Health amended its Sanitary Code to set a sixty eight degree minimum for apartments warmed by a central plant, after a winter of coal shortages in which landlords and tenants fought over how much heat was enough.⁵ That standard lives on in the Housing Maintenance Code.

Controls add to the problem. Steam systems generally switch on and off by the outdoor temperature rather than the temperature inside, so a building that has sealed its air leaks or replaced its windows can be overheated without the boiler registering it.¹ Urban Green lists the standard remedies together: master vents, indoor temperature sensors, thermostatic radiator valves on one pipe systems, and orifice plates that narrow the flow of steam into two pipe radiators.¹ Upgrades in older buildings come with a preservation caution. The Park Service recommends reusing radiator systems with new boilers where possible and keeping historic radiators that are significant elements of a building.³ The same balance between repair and replacement runs through the entry on renovating older buildings. City Limits reported in 2022 that a fatal Bronx fire that January, linked to a space heater, drew new attention to apartments left without adequate heat.⁷ In a Bronx building enrolled in the city’s heat sensor program, THE CITY reported in 2025, displaced residents said some tenants had used space heaters and stoves to stay warm.⁸